fix: enhance claim bot welcome message with 3-day PR reminder (JhaSourav07#896)

## Description

Fixes JhaSourav07#150 

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [ ] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

.github/scripts/issue-management/claim-handler.js

@@ -89,7 +89,7 @@ async function handleClaim({ github, context }) {
     owner,
     repo,
     issue_number: issueNumber,
-    body: `✅ Successfully assigned issue to @${commenter}\n\n> 💡 Please read [CONTRIBUTING.md](../blob/main/CONTRIBUTING.md) if you haven't already. Good luck! 🚀`,
+    body: `🎉 **Assigned!** Welcome to the project, @${commenter}.\n\n⏳ **Reminder:** You have **3 days** to submit a Pull Request. After 3 days of inactivity, you will be automatically unassigned to give others a chance (as per our GSSoC anti-hoarding policy).\n\n> 💡 Please read [CONTRIBUTING.md](../blob/main/CONTRIBUTING.md) if you haven't already.\n\nHappy coding! 🚀`,
   });
 }